1. Field of the Invention
This invention relates to the photorefractive transfer of energy between optical beams.
2. Description of the Related Art
Photorefractive materials have been used in a number of different applications involving the processing of optical beams. (The terms "light" and "optical" as used herein are not limited to the visible spectrum, but are used in their broader sense to include other regions of the spectrum such as the infrared). One principle application is in phase conjugate mirrors (PCMs). Other applications include optical switching, holography, image processing and the performance of optical mathematical functions such as image amplification, pattern substraction, and pattern recognition.
In general, a photorefractive (PR) material is one in which the index of refraction changes under the influence of applied light, such as a laser beam. The light causes charges within the PR material to migrate and separate, producing an internal electrostatic field. This field produces a change in the crystal's refractive index by the linear electro-optic (EO) effect (the Pockels effect). The theory of the EO effect is described in a text by A. Yariv, "Introduction to Optical Electronics, 2d ed.", pages 246-253 (1976). The PR index grating, or periodic variation in the crystal's index of refraction, is a measure of the change in the index. PR materials generally comprise III-V and II-VI semiconductor combinations within the periodic table, and other crystals such as BaTiO.sub.3, Bi.sub.12 SiO.sub.20 and KTa.sub.1-x NB.sub.x O.sub.3.
The formation of a PR index grating is illustrated in FIG. 1, in which the horizontal axis represents distance within the PR medium. The upper graph illustrates the pattern of light with a spatially periodic intensity I within the medium, while the next graph illustrates the resulting charge density. The mobile charges, illustrated as being of positive polarity, tend to accumulate in the dark regions of the light intensity pattern. The resulting periodic charge distribution produces a periodic electrostatic field E by Poisson's equation. This electric field, illustrated in the third graph of FIG. 1, then causes a change in the refractive index of the crystal by the linear EO effect. The index change is proportional to the EO coefficient and the space charge electrostatic field within the PR medium. The PR effect, illustrated in the last graph of FIG. 1, is nonlocal in that the maximum refractive index change does not occur at the peak of the light intensity. In FIG. 1 the spatial shift between the refractive index change and the intensity patter is 90.degree. with respect to the grating period; in general, however, this shift can be any fraction of the grating period.
Large energy transfers between optical beams are important in applications such as high contrast optical switches, and efficient self-pumped phase conjugators for laser power combining or aberration correction. The necessary degree of energy transfer has been possible previously using conventional EO photorefractivity in materials such as BaTiO.sub.3. These materials, however, have an undesirably slow response time. Furthermore, their sensitive wavelength region is in the visible, which is technologically less attractive than the near-infrared spectral region of diode and Nd:YAG lasers. Semi-insulating semiconductors, on the other hand, have a much faster response time and are compatible in wavelength with diode and Nd:YAG lasers. However, these semi-insulators do not exhibit sufficient photorefractivity to be useful, compared to BaTiO.sub.3, because of their small EO coefficient. Some photorefractivity enhancement in these materials has recently been reported using a DC electric field and moving gratings, or an AC electric field, as in Imbert et al., "High Photorefractive Gain in Two-Beam Coupling with Moving Fringes in GaAs:Cr Crystals", Optics Letters, Vol. 13, pages 327-329 (1988). The best reported net gain coefficient in semiconductors, however, has been only about 10 cm.sup.-1.
Another optical phenomenon of interest is the electrorefractive (ER) effect, also known as the FranzKeldysh effect. This is the change in absorption and refractive index of a semiconductor in the spectral region slightly smaller than the material's band gap. This effect has been measured in materials such as bulk InP and GaAs, as discussed in Van Eck, et al., "Franz-Keldysh Electrorefraction and Electroabsorption in Bulk InP and GaA", Applied Physics Letters, Vol. 48, No. 7, Feb. 17, 1986, pages 451-453. An earlier treatment of the ER effect in germanium and GaAs is given in Seraphin and Bottka, "Franz-Keldysh Effect of the Refractive Index in Semiconductors", Physical Review, Vol. 139, No. 2A, July 19, 1965, pages A560-A565.
The phenomenon is illustrated in simplified form in FIG. 2. The horizontal axis represents the photon energy of an applied optical beam (the beam energy varies inversely with its wavelength), while the vertical axis represents the material's absorption coefficient at each particular photon energy or wavelength. At an energy region E.sub.g, corresponding to the material's bandgap energy between the conduction and valence bands, the curve turns abruptly upward to become totally absorbing. If an electric field is imposed across the material, the absorption curve shifts to one of the modified curves 2 in the area just below E.sub.g, such that the transition becomes more gradual. The degree of shift from the basic absorption curve varies in accordance with the electric field strength. The region of variance near the absorption edge of the curve has been referred to as the "near-bandgap" region. This shift in absorption in the near-bandgap region is accompanied by a shift in the material's refractive index.
While of interest, investigations into the Franz-Keldysh effect have involved single optical beams, and have not been applicable to the current implementations described above for multiple-beam mixing. The investigations have been concerned with a region of high optical absorption, which further limits their application to practical systems.